Heartwood is wood that has become more resistant to decay as a result of deposition of chemical substances. Sapwood is the younger, outermost wood; in the growing tree it is living wood, and its principal functions are to conduct water from the roots to the leaves and to store up and give back according to the season the reserves prepared in the leaves. Mechanical Properties of Wood 1. Orthotropic Nature of Wood Wood may be described as an orthotropic material; that is, it has unique and independent mechanical properties in the directions of three mutually perpendicular axes: longitudinal, radial, and tangential. The longitudinal axis L is parallel to the fiber (grain); the radial axis R is normal to the growth rings (perpendicular to the grain in the radial direction); and the tangential axis T is perpendicular to the grain but tangent to the growth rings. 2. Elastic Properties a. Modulus of Elasticity Elasticity implies that deformations produced by low stress are completely recoverable after loads are removed. When loaded to higher stress levels, plastic deformation or failure occurs. The three moduli of elasticity, which are denoted by EL, ER, and ET, respectively, are the elastic moduli along the longitudinal, radial, and tangential axes of wood. These moduli are usually obtained from compression tests; however, data for ER and ET are not extensive. b. Modulus of Rigidity The modulus of rigidity, also called shear modulus, indicates the resistance to deflection of a member caused by shear stresses. c. Poisson’s Ratio When a member is loaded axially, the deformation perpendicular to the direction of the load is proportional to the deformation parallel to the direction of the load. The ratio of the transverse to axial strain is called Poisson’s ratio. 3. Strength Properties a. Modulus of rupture- Reflects the maximum load carrying capacity of a member in bending and is proportional to maximum moment borne by the specimen. Modulus of rupture is an accepted criterion of strength, although it is not a true stress because the formula by which it is computed is valid only to the elastic limit. b. Work to maximum load in bending- Ability to absorb shock with some permanent deformation and more or less injury to a specimen. Work to maximum load is a measure of the combined strength and toughness of wood under bending stresses. c. Compressive strength parallel to grain- Maximum stress sustained by a compression parallel-to-grain specimen having a ratio of length to least dimension of less than 11. d. Compressive stress perpendicular to grain- Reported as stress at proportional limit. There is no clearly defined ultimate stress for this property. e. Shear strength parallel to grain- Ability to resist internal slipping of one part upon another along the grain. Values presented are average strength in radial and tangential shear planes. f. Impact bending- In the impact bending test, a hammer of given weight is dropped upon a beam from successively increased heights until rupture occurs or the beam deflects 152 mm (6 in.) or more. The height of the maximum drop, or the drop that causes failure, is a comparative value that represents the ability of wood to absorb shocks that cause stresses beyond the proportional limit. g. Tensile strength perpendicular to grain- Resistance of wood to forces acting across the grain that tends to split a member. h.Hardness- Generally defined as resistance to indentation using a modified Janka hardness test, measured by the load required to embed a 11.28-mm (0.444-in.) ball to one-half its diameter. i. Tensile strength parallel to grain- Maximum tensile stress sustained in direction parallel to grain. 4. Other Properties a. Torsion strength- Resistance to twisting about a longitudinal axis. b. Toughness- Energy required to cause rapid complete failure in a centrally loaded bending specimen. c. Creep and duration of load- Time-dependent deformation of wood under load. If the load is sufficiently high and the duration of load is long, failure (creep–rupture) will eventually occur. The time required to reach rupture is commonly called duration of load. Duration of load is an important factor in setting design values for wood. d. Fatigue- Resistance to failure under specific combinations of cyclic loading conditions: frequency and number of cycles, maximum stress, ratio of maximum to minimum stress, and other less-important factors. e. Rolling shear strength- Shear strength of wood where shearing force is in a longitudinal plane and is acting perpendicular to the grain. f. Fracture toughness- Ability of wood to withstand flaws that initiate failure. Measurement of fracture toughness helps identify the length of critical flaws that initiate failure in materials. 5. Vibration Properties The vibration properties of primary interest in structural materials are speed of sound and internal friction (damping capacity). a. Speed of Sound The speed of sound in a structural material is a function of the modulus of elasticity and density. In wood, the speed of sound also varies with grain direction because the transverse modulus of elasticity is much less than the longitudinal value (as little as 1/20); the speed of sound across the grain is about one-fifth to one-third of the longitudinal value. b. Internal Friction When solid material is strained, some mechanical energy is dissipated as heat. Internal friction is the term used to denote the mechanism that causes this energy dissipation. The internal friction mechanism in wood is a complex function of temperature and moisture content. In general, there is a value of moisture content at which internal friction is minimum. On either side of this minimum, internal friction increases as moisture content varies down to zero or up to the fiber saturation point. Advantages of using timber as a structural material “Timber building is part of future energy-efficient building. Wood is sustainable, CO2 neutral and a highly effective insulator, creating excellent living conditions. One specific advantage of wood is its ability to reduce energy use. Timber construction has a higher heat insulation value than conventional construction methods, even with lower wall thicknesses. An external wall constructed using timber may have only half the thickness of a brick or concrete wall, yet provide double the thermal insulation value, while at the same time avoiding the thermal bridging common with other construction methods. Considering the growing importance of energy-efficient building methods, timber construction will play an increasingly important role in the future.” Wood is increasingly used in housing, nurseries and schools, religious, administrative, cultural and exhibition buildings, and halls and factories, as well as in transport-related construction like bridges, sound barriers, hydraulic engineering and avalanche control. The flexibility of lightweight modular timber construction is particularly suited to multi-purpose halls because of its ready adaptability. Wood is a high-performance material, low in weight, yet high in density, with excellent load-bearing and thermal properties, and the availability of a wide range of timbers, each with its own characteristics, means wood can be suitable for most special requirements. Timber construction is typically characterized by a multilayered combination of different materials which work together as a system to provide optimum stability, thermal, acoustic and moisture insulation, fire safety and constructional wood preservation. Flexibility The flexibility of timber construction methods makes it easier to vary a building’s orientation on site, its floor plan, the number of rooms, the interior design and the overall appearance, while timber’s thermal efficiency means walls can be slimmer, releasing up to 10% more space than other building methods. Durability With good design and correct detailing, structural wood needs no chemical treatment to achieve a long life. Wood is resistant to heat, frost, corrosion and pollution; the only factor that needs to be controlled is moisture. Timber construction materials are kiln- dried to specified moisture levels, removing the need for chemical wood treatment in interior use. Externally, design elements, such as large roof overhangs and sufficient distance between timber and ground are important. Timber facades are non-load bearing and therefore do not require treatment. However, extended life spans can be achieved by using heat treated timber, special timber qualities, treatments or decorative finishes.
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